For high speed electronic applications, the electrical interconnections are the bottleneck on a printed circuit boards (PCB) which limit data transfer rate between electronic components and from electronic components to the backplane. On the other hand, it can be expected that electronic components such as micro-processors or application specific integrated circuits (ASICs) become faster and faster in the next few years. An extrapolation of the clock rate of such electronic components plotted against a time scale is shown in FIG. 1 which is a diagram of the International Technology Roadmap for Semiconductors.
A widely accepted technical limit of the product of bitrate and distance for electrical interconnections is about 2.5 GBit/sec/meter. Therefore, higher transmission rates will be realized in the future with optical interconnection technology.
At present, there are different approaches known to realize optical interconnections. Some proposals are based on polymer planar multimode waveguides on a carrier foil which is able to be laminated within a standard multilayer PCB. This solution is quite near to the standard manufacturing process of a PCB and includes also surface mount technology for the board. A disadvantage is that such optical-electrical PCBs as well as surface mountable optical-electrical components are not yet available today, even not as research prototypes. It will thus take several years from now to develop these.
Another possible approach shown in FIG. 2 consists of a high speed board which has an optical overlay to realize the optical interconnections. High speed ASICs which process the data are closely located to the optical-electrical components such as Parallel Optical Links (POLs). The bitrate from and to the POLs can be up to 30 GBit/sec or even more. The optical overlay connects the POLs with either the backplane or makes interconnections between them.
In practice, the board will be manufactured in a standard process. In a further processing step, an optical overlay makes the optical interconnects as illustrated in FIG. 3, which is a picture from the company 3M. In this figure the overlay may be a foil with polymer waveguides but can also be a multiwire flexfoil, i.e., a flexible foil with optical fibers attached thereon. All components (POLs, Flexfoil, MT-Connector, and Optical Backplane connector for MT connector) needed for such an optical overlay are today available on the market as commercial products. Therefore, the shown optical interconnect on PCB via multiwire technology is available but is relative expensive, today. The reason for that is given in the mounting process of the MT-connector (MT: multi-fiber). For mounting an MT connector, the primary coating of the ribbon fiber has to be removed. However, an optical fiber without primary coating is very fragile which makes handling extremely difficult.